Settable composition for general purpose concrete and method of making same

ABSTRACT

From about 25% to about 50% but not more than about 200 lbs of the Portland cement of a general purpose concrete mix which contains for each cubic yard of the concrete to be produced in generally homogeneous admixture about 400-750 lbs. portland cement, about 1600-2000 lbs. coarse aggregate of a size of at least about 3/8 inch, and sufficient fine aggregate of a size less than 3/8 inch and up to minor amounts of conventional additives to yield one cubic yard when the admixture is combined with water in an amount equal to about 40-70% by weight of said portland cement, is replaced with a) Class F fly ash having a calcium oxide content up to about 6% and b) cement kiln dust (CKD) in a weight ratio of fly ash to CKD of about 2:3-3:2. Preferably, 30% or more of the Portland cement is replaced.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an continuation of application Ser. No. 07/946,785,filed Sep. 18, 1992, now abandoned, which is a continuation ofapplication Ser. No. 07/650,067, filed Feb. 7, 1991, now abandoned whichis a continuation-in-part of application Ser. No. 07/229,454, filed Aug.8, 1988, now U.S. Pat. No. 4,992,102.

FIELD OF THE INVENTION

This invention relates to the field of settable compositions for generalpurpose concrete construction containing Class F fly ash and cement kilndust as a substantial replacement for portland cement conventionallyused in such compositions.

BACKGROUND OF THE INVENTION

This invention is concerned with the utilization of two industrialby-products; namely, Class F fly ash and cement kiln dust (hereinafterCKD) in general purpose concrete-making composition. When finely dividedor pulverized coal is combusted at high temperatures, for example, inboilers for the steam generation of electricity, the ash consisting ofthe uncombustible residue plus a small amount of residual combustiblematter, is made up of two fractions, a bottom ash recovered from thefurnace or boiler in the form of a slag-like material and a fly ashwhich remains suspended in the flue gases from the combustion untilseparated therefrom by known separatory techniques, such aselectrostatic precipitation. This fly ash is an extremely finely dividedmaterial generally in the form of spherical bead-like particles, with atleast 70% by weight passing a 200 mesh sieve and has a generally glassystate, resulting from fusion or sintering during combustion. Asrecognized in the American Society of Testing Materials (ASTM)specification designation C618-85 entitled "Fly Ash and Raw or CalcinedNatural Pozzolan for Use as a Mineral Admixture in portland CementConcrete", fly ash is subdivided into two distinct classifications;namely, Class F and Class C. The definitions of these two classes are asfollows:

"Class F--Fly ash normally produced from burning anthracite orbituminous coal that meets the applicable requirements for this class asgiven herein. This class fly ash has pozzolanic properties.

Class C--Fly ash normally produced from lignite or subituminous coalthat meets the applicable requirements for this class as given herein.This class of fly ash, in addition to having pozzolanic properties, alsohas some cementitious properties. Some Class C fly ashes may containlime contents higher than 10%."

The latter reference to "pozzolanic properties" refers to the capabilityof certain mixtures which are not in themselves cementitious ofundergoing a cementitious reaction when mixed with lime in the presenceof water. Class C fly ash possesses direct cementitious properties aswell as pozzolanic properties. ASTM C618-85 is also applicable tonatural pozzolanic materials which are separately classified as Class Nbut are not pertinent here.

As the above quotation indicates, the type of coal to be combustedgenerally determines which class fly ash results, and the type of coalin turn is often dependent on its geographic origin. Thus, Class C flyash frequently results from coals mined in the Midwest; whereas Class Ffly ash often comes from coals mined in the Appalachian region. The ASTMspecification imposes certain "chemical requirements" upon therespective fly ash classifications thereof which are set forth below forthe relevant Classes F and C, including footnotes:

                  TABLE I-A                                                       ______________________________________                                        CHEMICAL REQUIREMENTS                                                                           Mineral Admixture Class                                                       F       C                                                   ______________________________________                                        Silicon dioxide (SiO.sup.2) plus aluminum                                                         70.0      50.0                                            oxide (Al.sub.2 O.sub.3) plus iron oxide                                      (Fe.sub.2 O.sub.3), min, %                                                    Sulfur trioxide (SO.sub.3), max, %                                                                5.0       5.0                                             Moisture content, max. %                                                                          3.0       3.0                                             Loss on ignition, max, %                                                                          .sup.  6.0.sup.1/                                                                       6.0                                             ______________________________________                                         .sup.1/ The use of Class F pozzolan containing up to 12.0% loss on            ignition may be approved by the user of either acceptable performance         records or laboratory test results are made available.                   

                  TABLE I-B                                                       ______________________________________                                        SUPPLEMENTARY OPTIONAL CHEMICAL                                               REQUIREMENT                                                                   Note: This optional requirement applies only when specifically                requested.                                                                                          F    C                                                  ______________________________________                                        Available alkalies, as Na.sub.2 O, max, %.sup.2/                                                      1.50   1.50                                           ______________________________________                                         .sup.2/ Applicable only when specifically required by the purchaser for       mineral admixture to be used in concrete containing reactive aggregate an     cement to meet a limitation on content of alkalies.                      

The ASTM physical requirements for both fly ash classes are virtuallythe same and are reproduced below exclusive of cautionary footnotes:

                  TABLE I-C                                                       ______________________________________                                        PHYSICAL REQUIREMENTS                                                                           Mineral Admixture Class                                                       F        C                                                  ______________________________________                                        Fineness:                                                                     Amount retained when wet-sieved on                                                                34         34                                             No. 325 (45 μm) sieve, max %                                               Pozzolanic activity index:                                                    With portland cement, at 28 days,                                                                 75         75                                             min. percent of control                                                       With lime, at 7 days min, psi (kPa)                                                               800        . . .                                                              (5500)                                                    Water requirement, max, percent of                                                                105        105                                            control                                                                       Soundness:                                                                    Autoclave expansion or contraction,                                                                 0.8        0.8                                          max %                                                                         Uniformity requirements:                                                      The specific gravity and fineness                                             of individual samples shall not                                               vary from the average                                                         established by the ten preceding                                              tests, or by all preceding tests                                              if the number is less than ten, by                                            more than:                                                                    Specific gravity, max variation                                                                    5          5                                             from average, %                                                               Percent retained on No. 325                                                                        5          5                                             (45 μm), max variation, per-                                               centage points from average                                                   ______________________________________                                    

CKD, on the other hand, is a by-product of the production of portlandcement clinkers by the high temperature furnacing of appropriate rawmaterials typically mixtures of limestone and clay or a low gradelimestone already containing a sufficient quantity of argillaceousmaterials often with added quantities of lime to adjust the finalcomposition. The resultant clinkers are pulverized by grinding, to ahigh degree of fineness and these particles upon admixture withsufficient water undergo a cementitious reaction and produce the solidproduct generally referred to as concrete, which exhibits highcompressive strength and is thus highly useful in construction of agreat variety of building or supporting structures. Generally, rotaryfurnaces are used for producing portland cement clinkers and a certainquantity of finely divided dust is produced as a by-product which iscarried off in the flue gases from such furnaces. The dust content canrange from about 5% of the clinkers output in so-called wet processplants up to as high as 15% in dry process plants. The suspended dust isremoved by various separating techniques and remains as a by-product ofthe cement making operation. Part of the CKD can be returned to thefurnace as recycled raw material, but it is not readily reincorporatedinto clinker formation and, in addition, tends to excessively elevatethe alkalinity of the ultimate portland cement.

The quantities of these two by-product materials which are producedannually are enormous and are likely only to increase in the future. Aspetroleum oil as the fuel for the generation of electricity is reducedbecause of conversation efforts and unfavorable economics vs. the morereadily available coal and as political considerations increasinglypreclude the construction of new nuclear power electrical generatingfacilities, or even the operation of already completed units of thistype, greater reliance will necessarily fall on coal as the fuel forgenerating electricity. As of 1979, the amount of CKD was estimated asaccumulating at a rate of 4-12 million tons per year in the UnitedStates alone; whereas the amount of Class F fly ash that is available isestimated to be about ten times what can be readily utilized. Obviously,there is an urgent growing need to find effective ways of employingthese unavoidable industrial by-products since otherwise they willcollect at a staggering rate and create crucial concerns over theiradverse environmental effect.

Various proposals have already been made for utilizing both fly ash andCKD. According to the text The Chemistry of Cement and Concrete by Lea,Chemical Publishing Company, Inc. 1971 edition, at page 421 et seq., flyash, i.e., Class F type, from boilers was first reported to bepotentially useful as a partial replacement for portland cement inconcrete construction about 50 years ago, and its utilization for thatpurpose has since become increasingly widespread. It is generallyaccepted that the proportion of portland cement replaced by the usualfly ash should not exceed about 20% to avoid significant reduction inthe compressive strength of the resultant concrete, although some morecautious jurisdictions may impose lower limits, e.g., the 15% maximumauthorized by the Virginia Department of Highways and Transportation(VDHT). As described in Lea at Page 437, the substitution of fly ashends to retard the early rate of hardening of the concrete so that theconcrete shows up to a 30% lower strength after seven days testing andup to a 25% lower strength after 28 days of testing, but in time thestrength levels equalize at replacement levels up to 20%. Increasing thesubstitution quantity up to 30% gives more drastic reduction in theearly compression values plus an ultimate reduction of at least about15% after one year.

The limited substitution of fly ash for portland cement in concreteformulations has other effects beyond compressive strength changes, bothpositive and negative. The fly ash tends to increase the workability ofthe cement mix and is recognized as desirably reducing the reactivity ofthe portland cement with so-called reactive aggregates. On the otherhand, fly ash contains a minor content of uncombusted carbon which actsto absorb air entrained in the concrete. Because entrained air increasesthe resistance of the hardened concrete to freezing, such reduction isundesirable but can be compensated for by the inclusion as an additiveof so-called air-entraining agents.

Utilization of fly ash for up to 20% of cement in concrete mixes at bestconsumes only a fraction of the available quantities of this material,and efforts have been made to increase its use. Dodson et al. in U.S.Pat. No. 4,210,457, while recognizing this accepted limit proposed thesubstitution of larger amounts of fly ash, and preferably more, of theportland cement with certain selected natural fly ashes having acombined content of silica, alumina and ferric oxide content, less than80% by weight, and a calcium content exceeding 10%, based on fivesamples of such ashes, varying from about 58-72% combined with a calciumoxide range of about 18-30%. Six other fly ash samples which are notsuitable at the high levels of 50% or more were shown to vary in thecombined oxide content from about 87-92% and in calcium oxide contentfrom about 4 to about 8%. Evaluating these values against the ASTMC618-85, one observes that the acceptable fly ashes came under the ClassC specifications, while the unacceptable ashes fell in the Class Fspecification. Thus, this patent in effect establishes that naturalClass C fly ashes are suitable for substantially higher levels ofreplacement for portland cement in concrete mixes than are Class F flyashes, and this capacity is now generally recognized, with Class C flyashes being generally permitted up to about a 50% replacement levelwhile maintaining the desirable physical properties of the concreteespecially compressive strength.

In U.S. Pat. No. 4,240,952, Hulbert, et al. while also acknowledging thegenerally recognized permissible limit of (Class F) fly ash replacementfor portland cement of 20%, proposed replacement of at lest 50% up to80%, provided the mix contained as additives about 2% of gypsum andabout 3% of calcium chloride by weight of the fly ash. The fly ashdescribed for this purpose, however, was a natural Class C fly ashanalyzing about 28% calcium oxide and combined silica, alumina andferric oxide content of about 63%. With up to 80% of this fly ash andthe specified additives, compressive strengths comparable to straightportland cement were said to be generally achievable. In one exampleusing 140 pounds portland cement and 560 pounds of fly ash (80-20 ratio)with conventional amounts of coarse and fine aggregate, and water andincluding the requisite additives, compressive strengths tested at 3180psi for 7 days, 4200 psi for 14 days and about 5000 psi at 28 days.

Obviously, the above patents cannot contribute to a solution to theproblem with Class F fly ash. In U.S. Pat. Nos. 4,018,617 and 4,101,332,Nicholson proposed the use of mixtures of fly ash (apparently Class F intype), cement kiln dust and aggregate for creating a stabilized basesupporting surface replacing conventional gravel-orasphalt-aggregate-stabilized bases in road construction wherein theuseful ranges were fly ash 6-24%, CKD 4-16% and aggregate 60-90%, with8% CKD, 12% fly ash and 80% aggregate preferred. Compressive strengthvalues for such measures as revealed in the examples varied rathererratically and generally exhibited only small increases in compressionstrength over the 7-28 day test period. Among the better results werefor the preferred mixture wherein the values increased from about 1100psi at 7 days to 1400 psi at 28 days. The addition of a small amount ofcalcium chloride added about a 200 psi increment to these values. On theother hand, the addition of 3% of lime stack dust recovered from a limekiln significantly reduced the results to about 700 psi at 7 days to900-1300 psi at 28 days. Elimination of the aggregate reduced thestrength to a fraction of the values otherwise, a mixture of 12% CKD and88% fly ash alone showing strength values of only about 190-260 psi overthe 28 day test period. Similarly, the choice of a finely dividedaggregate such as fill sand resulted in about the same fractional levelof strength values in the range of about 140-230 psi. A combination offinely divided and coarse aggregate in approximately equal amountsreduced the compressive strength values by about 1/2 with virtually nochange over the test period, giving values ranging from 650-750 psi,except where 1% of Type 1 portland cement was included which restoredthe strength values to about their general level, except at the initial7 days period where the strength values were about 800-900 psi increaseat 28 days to about 1200-1600 psi. Curiously, the best strength resultswere attained when 11.6% fly ash was combined with 3.4% lime with thebalance crushed aggregate, the CKD being omitted entirely, for which thestrength values while starting at a lower level of about 850-950 at 7days increased to about 1700 psi at 28 days.

The combination of fly ash and lime stack dust incidentally mentioned inthe later patent was explored further by Nicholson in U.S. Pat. No.4,038,095 which governs mixtures of about 10-14% fly ash, about 5-15%lime stack dust with the balance aggregate in the range of 71-85%.Somewhat inexplicably, the compressive results reported here for suchmixtures do not reach the high level specified in the first two patents,the strength values specified being only about 1000 psi with the moregeneral levels well below that depending on particular proportions.

In U.S. Pat. No. 4,268,316, Wills discloses the use of mixtures of kilndust and fly ash as a replacement for ground limestone and gypsum forforming a mortar or masonry cement, using proportions of about 25-55%portland cement, about 25-65% CKD and 10-25% fly ash. When these mortarformulations were mixed with damp sand in the proportions of about onepart cement mixture to 2.5-3 parts sand, compression strengthscomparable to those of standard masonry cement composed of 55% cementclinkers 40% limestone and 5% gypsum were shown for mixtures containing50% cement, 24-40% CKD and 15-25% fly ash. Inexplicably, in one example,when the cement content was increased to 55% with 35% CKD and 10-% flyash, the compressive strengths dropped by about 30-40% at both the 7 dayand 28 day ages to levels inferior to the standard material. As thecement content was decreased, with corresponding increases in the CKD,the compressive strength values dropped drastically. On the other hand,in another similar example mixtures containing 55% cement, 35% CKD and10% ash proved superior, particularly at the 28 day age, in compressivestrength to mixtures containing 50% cement, 35% fly ash and 15% CKD aswell as other standard masonry cements containing 50% cement, 47%limestone and 3% gypsum. Indeed, strength values dropped about 40% forthe mixtures, a 5% reduction in cement and a corresponding 5% increasein the fly ash to values definitely inferior to the standard cements.Similar variations were shown under laboratory test conditions forcomparable 50/35/15 mixtures dependent on the source of the fly ashwhile under actual construction conditions for the same mixtures,compressive strength values were reduced by about 50% for both theconventional masonry cement containing 55% portland cement andcomparable mixtures within the patented concept. The fly ash waspreferably Class F with Class C materials being less desirable.

In U.S. Pat. No. 4,407,677 Wills went on to teach that in themanufacture of concrete products such as blocks or bricks, the fly ashusually employed in combination with portland cement therein could bereplaced in its entirety by CKD with modes improvement in earlycompressive strength values for such products. Thus, at one day and twoday tests compressive strength values were shown of about 500-800 psi,but were said to increase to about 1200 psi after 28 days. The mixesdisclosed here contained 0.4-0.9 parts cement, about 0.1-0.6 parts CKDand 10-12 parts aggregate combining both fine and coarse materials, suchas expanded shale and naturel sand in a weight ratio of 80/20. Masonrycement generally develop at least about 95% of their strength propertiesat 28 days age so that additional aging of the patent products would notbe expected to result in any significant increase in their compressivestrength values.

In a different vein, an improved highly activated fly ash is obtained byMinnick in U.S. Pat. No. 3,643,115 by injecting lime together withbituminous coal into the combustion boiler to give a synthetic flay ashdeveloping early strength as high as five times that obtainedconventionally. The improved highly active fly ash can be mixed inproportions of 80-90 parts with 5-87 parts aggregate and 5-30 partswater. The injected lime combines with the sulfur dioxide releasedduring combustion of the coal, and additional sulfur may be needed ifthe coal has insufficient sulfur, giving a fly ash having a considerablyincreased sulfate content as well as calcium oxide and magnesiumcontents.

OBJECTS OF THE INVENTION

The object of the present invention is a settable composition for makinggeneral purpose concrete in which the cementitious ingredients thereofconsist essentially of from about 25% to about 50% by wt. up to amaximum of 200 lbs. of Class F fly ash and cement kiln dust.

A further object is a concrete mix of the general or all purpose varietyin which about 25% and preferably in excess of about 30% up to about 50%by weight of the portland cement used therein is replaced Class F flyash and cement kiln dust, in a weight ratio of fly ash to cement kilndust in the range of about 2:3-3:2.

SUMMARY OF RELATED INVENTION

The prior invention in application Ser. No. 07/229,454 was based on thediscovery that regular or common Class F fly ash having a combinedsilica, alumina and ferric oxide content of at least about 70% andpreferably at least about 80% by weight, as well as a calcium oxidecontent below about 10%, when blended homogeneously with cement kilndust (CKD) in the proportions of 60-40/40-60% by weight yields a blendhaving a combined silica, alumina and ferric oxide content of at lest50% but below about 70% with a calcium oxide content above about 10% andpreferably above about 10% and satisfies the essential specificationrequirements for a Class C fly ash of ASTM 1618-85 and thus constitutesa new synthetic Class C fly ash product. This product has been found tobe comparable with natural Class C fly ash in its important propertiesand especially for purposes of combination with portland cement forproducing general purpose concrete. In particular, it effectivelyreplaces the same large proportions of portland cement up to about 50%by weight as is generally approved for natural Class C fly ash incontrast to the substantially lower replacement levels allowed for ClassF fly ash. By replacing at least than about 25% and preferably more thanabout 30% of the portland cement up to the 50% limit, substantialeconomic savings can be achieved because of the favorable pricingstructure for the industrial waste products Class F fly ash and CKDcompared to the valuable and relatively expensive commodity portlandcement. For general or all purpose cement mixes, including so-calledready mixes as prepared and delivered in cement mixer trucks as well asbagged or bulk mixes for general or all purpose concrete construction,which would normally contain about 400-700 lbs. portland cement, about1600-2000 of a coarse aggregate and a sufficient quantity of a finegenerally sand-like aggregate plus minor amounts of any optionalingredients as to yield one cubic yard of concrete when admixed in waterequal in amount to about 40-70% weight of the portland cement, the newsynthetic Class C fly ash blend of this invention can replace at least25% and as much as 50% by weight of the portland cement up to a limit ofabout 200 lbs. wt., while effectively achieving substantially the samestructural properties, especially compressive strength in the resultantconcrete structure as achieved with 100% portland cement.

SUMMARY OF PRESENT VARIATION

Instead of combining the Class F fly ash and CKD in an admixture toproduce the new synthetic Class C fly ash contemplated in the priorrelated application Ser. No. 07/229,454, the same proportions of Class Ffly ash and CKD can be admixed separately with the other ingredients ofthe general purpose concrete mix by any convenient sequence. Because allof the mix ingredients are substantially dry and do not interact untilcombined with water during the concrete-making process, the ultimate mixcomposition as well as the properties of the general purpose concreteproduced therefrom are exactly the same irrespective of whether theClass F fly ash and CKD are incorporated into the mix separately orpreliminarily combined together into a blend which is then added to theother ingredients, provided of course that the proportion of allingredients remain the same. The only difference here is the need forseparate storage and delivery facilities at the mix plant for the ClassF fly ash and CKD individually whereas the prior concept envisioned thepurchase of the new synthetic Class C fly ash as a new commodity by themix manufacturer or ready mix operator which then needed only onestorage facility.

The sequence of addition of the ingredients for present purposes isimmaterial and can be in any order desired for efficient mixing. Theingredients can be loaded one by one directly into a ready-mix truck orcan be combined for handling in bulk, including in bags.

The Class F fly ash and CKD are used in the relative weight ration inthe range of about 3:2-2:3. The amounts of other ingredients are thesame as before.

DETAILED DESCRIPTION OF THE INVENTION

Any standard or common Class F fly ash obtained from boilers and likefurnaces used for the combustion of pulverized coal, particularly of abituminous or anthracite type, and especially from coal-fired,steam-generating plants of electrical utilities, is suitable for use asthe Class F fly ash component of this invention. Such fly ash shouldhave a combined silica, alumina and ferric oxide content of at leastabout 70% and preferably 80% or higher by weight and a calcium oxidecontent below about 10%, usually about 6% by weight or less. A specificfly ash found to give particularly good result sin the invention isso-called "Carbo" fly ash obtained from the Clinch River Power of theAmerican Electric Power Service Corporation at Carbo, Va. This specificfly ash is available from Ash Management Corp., a subsidiary of JTMIndustries of Marietta, Ga. An analysis of this preferred Carbo fly ashappears in the following Table II which sets forth the chemicalcomposition and certain physical properties of fly ash samples takenperiodically from this plant over a nine month period.

                                      TABLE II                                    __________________________________________________________________________    ANALYSIS OF TYPICAL CLASS F FLY ASH                                                        Sample                                                                        1  2  3  4  5  6  7  8  9  10                                    __________________________________________________________________________    CHEMICAL ANALYSIS                                                             Silica       50.0                                                                             50.7                                                                             51.2                                                                             53.0                                                                             53.0                                                                             49.4                                                                             50.6                                                                             50.9                                                                             56.3                                                                             54.5                                  Aluminum Oxide                                                                             29.8                                                                             28.5                                                                             25.5                                                                             26.1                                                                             24.7                                                                             25.4                                                                             25.2                                                                             24.6                                                                             23.1                                                                             24.8                                  Iron Oxide   6.8                                                                              7.0                                                                              7.6                                                                              7.8                                                                              7.4                                                                              8.4                                                                              8.2                                                                              8.9                                                                              7.7                                                                              7.3                                   Combined 1,2,3                                                                             86.6                                                                             86.2                                                                             84.3                                                                             86.9                                                                             85.1                                                                             83.2                                                                             84.0                                                                             83.5                                                                             87.1                                                                             86.6                                  Titanium Oxide                                                                             1.6                                                                              1.6                                                                              1.6                                                                              1.7                                                                              1.7                                                                              1.5                                                                              1.5                                                                              1.5                                                                              1.5                                                                              1.7                                   Calcium Oxide                                                                              6.1                                                                              6.3                                                                              6.6                                                                              4.1                                                                              5.7                                                                              7.0                                                                              9.1                                                                              6.8                                                                              5.8                                                                              5.6                                   Magnesium Oxide                                                                            1.7                                                                              1.7                                                                              1.8                                                                              1.6                                                                              1.6                                                                              1.8                                                                              2.0                                                                              1.8                                                                              1.4                                                                              1.6                                   Sodium Oxide 0.8                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.7                                                                              0.6                                                                              0.7                                                                              0.6                                                                              0.6                                                                              0.6                                   Potassium Oxide                                                                            2.5                                                                              2.8                                                                              2.6                                                                              2.9                                                                              2.4                                                                              2.8                                                                              2.5                                                                              2.9                                                                              2.7                                                                              2.6                                   Sulfur Trioxide                                                                            0.9                                                                              0.8                                                                              0.7                                                                              0.5                                                                              0.6                                                                              0.9                                                                              1.0                                                                              0.9                                                                              0.9                                                                              0.8                                   Phosphorus Pentoxide                                                                       0.3                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.3                                                                              0.3                                                                              0.2                                   Other Constituents                                                                         1.2                                                                              1.0                                                                              1.1                                                                              0.7                                                                              0.7                                                                              0.6                                                                              0.6                                                                              0.7                                                                              0.7                                                                              1.0                                   Available Alkalies, %                                                                      .65                                                                              .78                                                                              .77                                                                              .76                                                                              .66                                                                              .78                                                                              .76                                                                              .70                                                                              .78                                                                              .83                                   Averaged Combined 1, 2, & 3 = 85.3; av. CaO = 6.21                            PHYSICAL TESTS                                                                Moisture Content, %                                                                        .02                                                                              .14                                                                              .05                                                                              .03                                                                              .02                                                                              .18                                                                              .05                                                                              .01                                                                              .12                                      Neg Ignition Loss                                                                          1.07                                                                             1.27                                                                             1.01                                                                             0.94                                                                             0.60                                                                             0.76                                                                             0.62                                                                             0.82                                                                             0.75                                     Retained on No. 325,                                                                       19.99                                                                            21.68                                                                            20.39                                                                            20.58                                                                            19.34                                                                            17.66                                                                            19.13                                                                            22.53                                                                            20.80                                    Wet-Sieved, %                                                                 Specific Gravity                                                                           2.27                                                                             2.24                                                                             2.22                                                                             2.25  2.26                                                                             2.26                                                                             2.27                                                                             2.26                                     __________________________________________________________________________

Identification of other common Class F fly ashes can be found throughoutthe literature. For example, U.S. Pat. No. 4,210,457 mentionedpreviously, provides an analysis in Table I thereof for six samples ofClass F fly ashes, identified F-K from which the content of constituentsthereof expressed as a weight percent range and an average value aresummarized in the following Table III.

                  TABLE III                                                       ______________________________________                                        VARIOUS CLASS F FLY ASHES FROM USP 4,210,457                                                 Main Components, % wt.                                                        Range   Average                                                ______________________________________                                        SiO.sub.2        43.3-56.3 49.3                                               Al.sub.2 O.sub.3 18.5-31.0  24.65                                             Fe.sub.2 O.sub.3  5.6-29.9 14.2                                               CaO              4.3-7.7    5.95                                              SiO.sub.2 + Al.sub.2 O.sub.3 + Fe.sub.2 O.sub.3                                                93.9-91.7 88.1                                               ______________________________________                                    

In addition, the Lea text identified above gives in Table 71 at Page 422an analysis for four Class F fly ashes, two from the USA and two fromGreat Britain. Similarly, analysis for representative fly ashes, bothClass F and Class C, from Texas sources appear at pages 175-178 inResearch Report 450-1, project 3-9-85-450 by Center for TransportationResearch, Bureau of Engineering Research, the University of Texas atAustin, entitled "Effectiveness of Fly Ash Replacement in the Reductionof Damage Due to Alkali-Aggregate Reaction in Concrete" by Farbiarz andCarrasquillo, May 1986. In this report, the Class F fly ashes areidentified as "Type A"; while the Class C fly ashes are identified as"Type B" according to whether the fly ashes contain a high content ofcombined silica, alumina and ferric oxide with low calcium oxide or viceversa. One sample described in this report illustrates the unreliabilityof a classification of fly ashes along geological lines, i.e., whetherderived from bituminous and anthracite coal versus subituminous andlignite coal, that sample being considered a Type B (Class C) ashdespite its derivation from sublignite and a normal classification byits supplier as Class F.

Similarly, any common cement kiln dust (CKD) which is produced as aby-product during the industrial production of portland cement would inprinciple be suitable for purposes of this invention. One specific CKD,obtained as a matter of convenience, from the Tarmac Lone Star CementCompany cement plant at Roanoke, Va., has been found entirely useful inthe invention, and an analysis of this CKD is set forth in the followingTable IV.

                  TABLE IV                                                        ______________________________________                                        ANALYSIS OF TYPICAL CEMENT KILN DUST (CKD)                                    ______________________________________                                        Chemical Analysis, %                                                          Insoluble Residue                                                             ______________________________________                                        SiO.sub.2       16.80                                                         Al.sub.2 O.sub.3                                                                              5.12                                                          Fe.sub.2 O.sub.3                                                                              2.38                                                          CaO             45.89                                                         MgO             2.22                                                          SO.sub.3        4.93                                                          K.sub.2 O       2.65                                                          Na.sub.2 O      40.00                                                         Total Alkali    2.14                                                          Combine 1, 2, & 3                                                                             24.3                                                          Loss of Ignition                                                                              19.60                                                         ______________________________________                                        Particle Size                                                                 Sieve #         % Passing                                                     ______________________________________                                         20             98.6                                                           50             96.5                                                          100             93.8                                                          200             85.4                                                          325             74.6                                                          ______________________________________                                    

Composition for various samples of CKD can be found in the patentliterature wherein, for example, nine different samples have beenanalyzed by Nicholson in U.S. Pat. No. 4,018,617 mentioned previously. Asummary of the range and average of the weight percent values forcontents of the main components of these nine samples as taken from thispatent appears below as Table V.

                  TABLE V                                                         ______________________________________                                        VARIOUS CKD'S FROM U.S. Pat. No. 4,019,617                                    Main Components, % wt                                                                    Range                                                              Ingredient   Low         High   Average                                       ______________________________________                                        SiO.sub.2    6.0         28.5   16.5                                          Al.sub.2 O.sub.3                                                                           3.2          9.6    4.35                                         Fe.sub.2 O.sub.3                                                                           0.8          5.9    2.66                                         CaO          16.0        65.0   47.6                                          Combine 1, 2, & 3                                                                          10.0        44.0   23.5                                          ______________________________________                                    

As is known, Class F fly ashes generally contain a certain amount ofelemental carbon resulting from incomplete combustion of the pulverizedcoal and because of the undesirable effects of the carbon contentthereof on the air entrainment capability of concrete prepared frommixes containing the same, as stated before, the amount of such carbonis ordinarily restricted. An upper limit of 6% is common for manyjurisdictions; although others such as the VDHT permit only 2.5% carbon.On the other hand, certain countries, such as Canada, allow up to 12%carbon. Because in the practice of the invention, the Class F fly ash isemployed in fairly minor amounts, the effective amount of carboncontained in the ultimate overall mix is proportionately reduced fromthe amount in the starting Class F fly ash whereby higher fly ashcontents in the latter are tolerable. Thus, the carbon content of thepresent compositions is well within even the lower permissible limitsand often is negligibly small.

As will be established later, within the above limits for thecompositions of the invention, the concrete produced therefrom exhibitsubstantially comparable properties for use in general purpose cementconstruction, especially compressive strength to corresponding allportland cement mixes. This being the case, economic considerations maybe an important factor in selecting a specific mix within such ranges.Under present market conditions, and dependent upon transportationdistances from the available sources of the two components, CKD can bepurchased somewhat more cheaply than can a standard Class F fly ash. Forexample, fly ash might be purchased at a cost of $20.00 per tonincluding transportation expense of about $7.00 per ton; whereas CKD canbe purchased for about $9.00 per ton including about $4.00transportation expense. Where the relative expense significantly favorsone of the products, such as CKD, it is economically advantageous toutilize a larger amount of the cheaper constituent. Thus a mix having aweight ratio of Class F fly ash to CKD of 2:3 would be cheaper toproduce than a ratio of 3:2.

The choice of aggregate material for concrete mixes using the presentblends will pose not problem to the person skilled in the design of suchmixes. The coarse aggregate should have a minimum size of about 3/8 inchand can vary in size from that minimum up to one inch or larger,preferably in gradations between these limits. Crushed limestone, graveland the like are desirable coarse aggregates, and the material selectedin any case should exhibit a considerable hardness and durabilityinasmuch as crumbly, friable aggregates tend to significantly reduce thestrength of the ultimate concrete. The finely divided aggregate issmaller than 3/8 inch in size and again is preferably graduated in muchfiner sizes down to 200 sieve size or so. Ground limestone, sand and thelike are common useful fine aggregates.

Several different types of portland cement are available and all are inprinciple, useful. Type I is the general purpose variety and is mostcommonly employed but Type III can be substituted if early strengthdevelopment is desirable. Commercial blended cements, such as Type I-P,wherein 20% Class F fly ash is blended with 80% by weight portlandcement clinker during pulverization should be avoided.

The mixes of the invention are prepared by homogeneously and uniformlymixing all of the mix ingredients including the Class F fly ash and CKD.The Class F fly ash has a specific gravity of about 2.25, while that ofCKD is around 2.70. These relatively small differences in specificgravities does not create any unusual problems in the preparation of thepresent compositions and any of the usual mixing techniques commonlyemployed in the concrete mix industry as suitable. The ultimatecompositions are no more susceptible to undergo separation duringhandling and storage than are ordinarily concrete mixes. They can betransported and stored in the same manner as the ordinary mixes as canthe individual ingredients. The storage containers should, of course, beclosed to protect the contents thereof from weather.

In formulating specific mixes for concrete construction, referenceshould be made to the procedures established therefor by the AmericanConcrete Industry (ACI). As is well known by those knowledgeable in thearea, ACI concrete design mixes are calculated on an absolute volumebasis to give a cubic yard or 27 ft³ of the desired concrete. Byabsolute volume is meant the theoretical volume of each constituent ofthe mix relative to the volume of 1 ft³ of water. By knowing specificgravity of each of the various constituent materials, one can readilycalculate the absolute volume thereof independently of actual densityvariations. In the industry, concrete mixes are arbitrarily identifiedin terms of certain even-numbered levels of compression strength, suchas a "3000# concrete", a "4000# concrete", etc. By usage, the actualconcrete rated at the respective levels is expected to possess an actualcompressive strength substantially higher, usually by an increment ofabout 1200 psi, than the rated level, Thus, a "3000# concrete" would beexpected to have an actual compression strength of at least 4200 psi,while a "6000# concrete" would have an actual compression strength of atleast 7200 psi. The industry also refers to cement mixes in terms of thenumber of bags of portland cement, each of 94 lb. weight, that would beutilized to give a cubic yard of the mix, the bag number being knownfrom experience to have a rough correlation with a given compressionstrength. Thus, a "4 bag mix" would roughly equate with a "2500#concrete" and a "7.5 bag mix" with a "6000# concrete". This correlationis only general at best since the actual compression strength exhibitedby any specific concrete mix design can only be determined by actualexperimental testing.

A number of design mixes based on the synthetic Class C fly ash blendsof the prior application Ser. No. 07/227,454 were prepared and analyzedand tested according to accepted standards as established by ASTM andACI, and the results of these analyses and tests are set for the in thetabulations of the examples. The concrete design mixes specified thereinwere devised using the ACI procedure. Preferably, the concrete mixes ofthe invention would contain per cubic yard portland cement in the rangeof about 400-700# before any replacement thereof with Class F fly ashand CKD covering the range of about a 4.0 bag mix to an 8.0 bag mix.Concrete with less than about 400 lb. portland cement tends to be quiteweak and has only limited usefulness. On the other hand, concretecontaining per cubic yard more than 8 bags of cement are known but arespecialized products exhibiting extremely high levels of compressivestrength and specialized knowledge is needed in the formulation andutilization of such mixes. In the present compositions, about 25-50% ofthe portland cement can be replaced up to a maximum replacement ofroughly about 200 lbs. or so of portland cement in order to avoid riskof loss in compression strength with at the higher "bag" levels ofportland cement needed for high compressive strength concrete mixes.That is, for a 4 bag mix, about 50% of portland cement could be replacedwhile for a 7.5-8 bag ix, the amount of replaced portland cement shouldbe limited to about 200# or about 26-28% because if the content ofportland cement were reduced proportionately in the higher bag mixes,the concrete produced therefrom would suffer in compression strength toan undesirable degree.

The amount of the coarse aggregate used in mixes containing this rangeof portland cement would be in the rage of about 1600-2000#, dependentmainly with the so-called fineness modulus of the fine aggregate asdetermined by the supplier thereof and applying an established ACIcorrelation. This weight range is based on a unit weight of coarseaggregate of 100#/ft³, a fineness modular of 2.4-2.7 and a volume % ofcoarse aggregate per yard³ of concrete of 0-/5-0.7 for a maximumaggregate size of 1/2-1". Coarse aggregate can vary in unit weightgenerally within a range of 90-100#/ft³ and the above range can varyaccordingly about ±10%. The amount of the water needed for admixturewith such mixes to make good concrete would vary between about 40-70% byweight of the total of cementitious materials present including theClass F fly ash and CKD as well as portland cement. As the examplesshow, the inclusion of small amounts of optional additives is customary,particularly such additives as air entraining agents and water reducingagents, all of which are known and need no further description here.After the collective amount of these several materials has beendetermined, the balance of he mix to make one cubic yard is constitutedby the fine aggregate such as sand, and this amount is adjusted toreflect changes in the quantities of the other ingredients. That is, thetotal absolute volume of all ingredients other than fine aggregate,including the volume of air allocated by design specification forentrained air, is subtracted from 27 ft³ to give the ft³ to be suppliedby fine aggregate. The difference in ft³ can be converted to weight bymultiplying it by the specific gravity of the fine aggregate times thewt/ft³ of water (62.4#). It will, of course, be understood that thedosage rates specified in the exemplary mix designs, particularly forthe additives such as the air entraining agent, etc., may needadjustment due to changes in temperature and other local factors.

The results in the following examples were actually obtained bypreliminarily blending, in each case, the Class F fly ash and CKDtogether in accordance with the concept of the prior application andcombining the blend with the other mix ingredients. However, the resultswould be identical if the same proportionate amount for each of theClass F fly ash and CKD was added separately to the remaining mixingredients and the proportionate amounts of the Class F fly ash and CKDhave been expressed in each case in terms of their relative weight ratioof the particular mix.

                                      EXAMPLE 1                                   __________________________________________________________________________    Concrete Mix Designs, Air Entrainment Type, with Class F Fly Ash and CKD      In Weight Ratio of 1:14, 42% Class F Fly Ash, 58% CKD by Weight                                   Mix D-5                                                                            Mix D-6                                                                            Mix D-7                                                                            Mix D-8                                                        (4.5 bags)                                                                         (5.0 bags)                                                                         (5.5 bags)                                                                         (6.7 bags)                                 __________________________________________________________________________    Santee Cement I, pounds                                                                           236  284  338  526                                        Class F Fly Ash, pounds                                                                           79   78   75   44                                         CKD, pounds         110  108  103  61                                         Total Cementitious Materials, lbs.                                                                425  470  516  631                                        % of Above Fly Ash and CKD combined                                                               44.5 39.6 34.5 16.6                                       Cormix ARE Agent, ounces                                                                          6.0  8.0  10.0 9.0                                        Cormix Water Reducer, ounces                                                                      22.1 23.6 25.9 31.7                                       #67 Vulcan Stone, pounds                                                                          1750 1750 1750 1750                                       Vulcan Concrete Sand, pounds                                                                      1492 1468 1428 1337                                       Total Mix Water, Gallons                                                                          33.0 33.0 33.0 33.0                                       Water-Cement Ratio  0.65 0.58 0.53 0.44                                       (All cementitious materials)                                                  Slump, inches       2.5  4.25 3.25 3.25                                       Air Content, percent                                                                              4.5  5.9  4.1  5.2                                        Temperature of concrete F                                                                         83   83   84   85                                         Temperature of air, F                                                                             85   84   84   84                                         Unit Weight, lbs/cu ft                                                                            147.0                                                                              147.4                                                                              147.2                                                                              148.1                                      COMPRESSIVE STRENGTH, PSI                                                     7-Day Test Results   2441                                                                              2618 3520 4950                                                           2480 2564 3467 5005                                       Average             2461 2591 3494 4978                                       28-Day Test Results 3500 3714 4320 5730                                                           3400 3767 4369 5836                                                           3431 3784 4240 5889                                       Average             3444 3755 4310 5818                                       56-Day Test Results 4070 4369 5005 6013                                       __________________________________________________________________________    AGGREGATE DATA                                                                                   % PASSING                                                  Sieve Size         Fine Aggregate                                                                        Coarse Aggregate                                   __________________________________________________________________________    1"                         100.0                                              3/4"                       87.9                                               1/2"                       28.3                                               3/8"               100      6.6                                               #4                 99.8     1.3                                               #8                 85.3     0.8                                               #16                60.0                                                       #30                44.2                                                       #50                28.1                                                       #100               15.2                                                       #200                5.1    Trace                                              Fineness Modulus    2.67   --                                                 Specific Gravity    2.93    2.80                                              Absorption, percent                                                                               0.30    0.40                                              Dry Rodded Unit Weight, pct                                                                      121.5   96.40                                              __________________________________________________________________________

Mix D-6 of Example 1 above (containing 284# portland cement) wascompared in the field with a similar conventional mix formulatedaccording to the same mix design protocol except for the use in thecommercial mix of 400# of portland cement plus 100# of a typical Class Ffly ash (20% replacement level) for a total weight of cementitiousmaterials of 500#, this mix being rated as a "3000# cement". Samplestaken in the field were tested for compression strength usingstandardized techniques and the resultant values are compared below.

                  Example 2                                                       ______________________________________                                        Comparison of Mix D-6 of Example 1                                            with Similar Conventional Mix                                                            Compressive Strength Result                                                   at 7 days                                                                             at 28 days at 56 days                                      ______________________________________                                        Mix D-6      2280      3465       4225                                        Regular 3000# Mix                                                                          2270      3600       4240                                        ______________________________________                                    

It will be noted that the above values do not correspond exactly withthe values for Mix D-6 as specified in Example 1. The latter values weredetermined under laboratory conditions, and as is to be expected smallvariations in such values of about plus or minus 10% are entirelynormal.

                  Example 3                                                       ______________________________________                                        Concrete Mix Designs, Air Entrainment Type, with                              Class F Fly Ash and CKD at weight Ratio of 1:1.4                                              Mix C-1   Mix C-2  Mix C-3                                                    (4.5 bag) (5.0 bag)                                                                              (6.0 bag)                                  ______________________________________                                        LeHigh Cement II, lbs.                                                                        240       280      374                                        Class F Fly Ash, lbs.                                                                         80        80       80                                         CKD, lbs.       110       110      110                                        Total Cementitious Material,                                                                  430       470      564                                        lbs.                                                                          % of Above for Fly Ash and                                                                    44.2      40.4     33.7                                       CKD combined                                                                  Pozzolith 322-N, ozs.                                                                         17.2      18.8     22.6                                       Micro Air, ARE, Agent, ozs.                                                                   2.0       2.2      2.6                                        Limon Springs Natural Sand                                                                    1298      1267     1188                                       Vulcan #68 Stone, lbs.                                                                        1750      1750     1750                                       Total Mix Water, gals                                                                         34.5      34.5     34.5                                       Water-Cement Ratio (All                                                                       0.64      0.61     0.51                                       Cementitious Materials)                                                       Slump, inches   4.0       3.5      4.0                                        Air Content, Percent                                                                          5.8       5.0      5.2                                        Temperature of concrete, F                                                                    76        73       72                                         Temperature of Air, F                                                                         76        77       77                                         Unit Weight, lbs/cu. ft.                                                                      142.9     144.9    145.9                                      COMPRESSIVE STRENGTH TEST RESULTS                                             7-Day Test Results                                                                            1981      2829     3678                                                       2016      2688     3749                                       Average         1998      2758     3714                                       28-Day Test Results                                                                           3395      4421     5376                                                       3395      4739     5516                                       Average         3395      4580     5446                                       56-Day Test Results                                                                           4173      4952     6083                                                       4244      5164     6296                                       Average         4208      5058     6190                                       ______________________________________                                        TEST DATA ON AGGREGATES                                                                 PERCENT PASSING                                                                 Fine Aggregate Coarse Aggregate                                   Sieve Size  Limon Springs Sand                                                                           Vulcan #68 Stone                                   ______________________________________                                        1"                         100.0                                              3/4"                       87.9                                               1/2"                       28.3                                               3/8"                       6.6                                                #4          100.0          1.3                                                #8          98.0           0.8                                                #16         82.0                                                              #30         45.0                                                              #50         20.0                                                              #100        3.0                                                               #200        1.5                                                               Fineness Modulus                                                                          2.53           --                                                 Absorption, Percent                                                                       1.0            0.41                                               Specific Gravity                                                                          2.64           2.62                                               Dry Rodded Unit            99.48                                              Weight, pcf 97.48                                                             ______________________________________                                    

                  Example 4                                                       ______________________________________                                        Plain Mix Design Using 42% Class F Fly Ash and                                CKD in Weight Ratio of 1:1.4                                                                  Mix PL-1  Mix PL-2 Mix PL-3                                                   (4.5 bag) (6.5 bag)                                                                              (7.5 bag)                                  ______________________________________                                        Lone Star Cement II, lbs.                                                                     327       421      515                                        Class F Fly Ash, lbs                                                                          80        80       80                                         CKD, lbs        110       110      110                                        Total Cementitious, lbs.                                                                      517       611      705                                        % Cement Replaced                                                                             37.75     31.1     27                                         Pozzolith 133-N.sup.1/, oz.                                                                   20.7      24.4     28.2                                       Martinsville #57, Stone, lbs.                                                                 1870      1870     1870                                       Madison Natural Sand, lbs.                                                                    1330      1250     1175                                       Total Mix Water, gals.                                                                        40.0      40.0     40.0                                       Water-Cement Ratio (All                                                                       0.645     0.55     0.473                                      Cementitious Materials)                                                       Slump, inches   3.50      3.75     4.25                                       Unit Weight, lbs/cu. ft.                                                                      151.2     151.1    152.2                                      Temperature of concrete, F                                                                    81        80       71                                         Temperature of Air, F                                                                         82        82       71                                         COMPRESSIVE STRENGTH TEST RESULTS                                             7-Day Test Results                                                                            2546      3183     4315                                                       2617      3183     4244                                       Average         2582      3183     4280                                       28-Day Test Results                                                                           4067      4598     5659                                                       3926      4386     5730                                       Average         3997      4492     5695                                       56-Day Test Results                                                                           4244      5199     6367                                                       4386      5128     6084                                       Average         4315      5164     6226                                       ______________________________________                                        AGGREGATE DATA                                                                                   % PASSING                                                  Sieve Size           Fine    Coarse                                           ______________________________________                                        1"                           100.0                                            3/4"                         73.5                                             1/2"                         25.5                                             3/8"                 100.0   5.1                                              #4                   99.9    1.6                                              #8                   95.7    1.3                                              #16                  81.1                                                     #30                  44.0                                                     #50                  12.3                                                     #100                  2.5                                                     Fineness Modulus      2.65   --                                               Absorption, Percent   1.50    0.35                                            Specific Gravity      2.61    2.81                                            Dry Rodded Unit      94.4    103.4                                            Weight, pcf                                                                   Material Finder than 200 mesh                                                                       0.6    trace                                            ______________________________________                                         .sup.1/ a commercial waterreducing agent                                 

                  Example 5                                                       ______________________________________                                        Air Entrainment Mix Using 42% Class F Fly Ash and                             CKD in Weight Ratio of 1:1.4                                                                 Mix ARE-1 Mix ARE-2 Mix ARE-3                                                 (4.5 bag) (5.0 bag) (6.0 bag)                                  ______________________________________                                        Lone Star Cement II, lbs.                                                                    327       421       515                                        Class F Fly Ash, lbs.                                                                        80        80        80                                         CKD, lbs.      110       110       110                                        Total Cementitious, lbs.                                                                     517       611       705                                        % Cement Replaced                                                                            37.75     31.1      27                                         MBVR-C ARE Agent,.sup.1/                                                                     3.0       4.0       5.0                                        ozs.                                                                          Pozzolith 133-N, oz.                                                                         20.7      24.4      28.2                                       Martinsville #57, Stone,                                                                     1870      1870      1870                                       lbs.                                                                          Madison Natural Sand, lbs.                                                                   1240      1165      1085                                       Total Mix Water, gals.                                                                       34.0      34.0      34.0                                       Water-Cement Ratio (All                                                       Cementitious Materials)                                                                      0.55      0.46      0.40                                       Slump, inches  4.00      3.75      3.50                                       Unit Weight, lbs/cu. ft.                                                                     151.2     151.1     152.2                                      Air Content, % 4.5       5.0       4.7                                        Temperature of 76        80        73                                         concrete, F                                                                   Temperature of Air, F                                                                        78        82        72                                         COMPRESSIVE STRENGTH TEST RESULTS                                             7-Day Test Results                                                                           2688      3395      4386                                                      2688      3395      4456                                       Average        2688      3395      4421                                       28-Day Test Results                                                                          4386      4775      5800                                                      3820      4881      5871                                       Average        4103      4828      5836                                       56-Day Test Results                                                                          4492      5376      6225                                                       4633     5341      6225                                       Average        4563      5359      .sup.  6225.sup.2/                         ______________________________________                                         .sup.1/ Commercial Air Entraining Agent                                       .sup.2/ Values are low due to weakness of above aggregate                     Aggregate Data same as Example 4                                         

                  Example 6                                                       ______________________________________                                        Air-Entrainment Mix Designs Using Various Weight Ratios                       of Class F Fly Ash and CKD                                                                           Mix FCl                                                ______________________________________                                        Lone Star Cement I, lbs.                                                                             280                                                    Total Class F Fly Ash and CKD, lbs.                                                                  190                                                    Daravair ARE Agent, ozs.                                                                             5.0                                                    WRDA Water Reducer, ozs.                                                                             20.0                                                   #5 Rockydale Stone, lbs.                                                                             1110.0                                                 #7 Rockydale Stone. lbs.                                                                             740.0                                                  Lone Star Natural Sand, lbs.                                                                         929.0                                                  Castle Sand, lbs.      394.0                                                  Total Mix Water, gals  33.0                                                   Water-Cement Ratio     0.58                                                   Concrete Temperature, F.                                                                             67-69                                                  Air Temperature, F.    70-72                                                  ______________________________________                                        TEST DATA ON AGGREGATES                                                                    Percent Passing                                                                 Fine Aggregate                                                                            Coarse Aggregate                                   Sieve size     70/30 Blend 60/40 Blend                                        ______________________________________                                        1 1/2"                     100.0                                              1"                         95.2                                               3/4"                       70.1                                               1/2"                       47.0                                               3/8"                       29.9                                               #4             100.0                                                          #8             97.8                                                           #16            92.6                                                           #30            80.9                                                           #50            47.5                                                           #100           11.5                                                           #200           2.1                                                            Fineness Modulus                                                                             2.68        --                                                 Absorption, % LS1                                                                            0.5         0.4                                                0.4 Castle                                                                    Specific Gravity                                                                             2.61        2.80                                               LS1 2.64 Castle                                                               ______________________________________                                        Test Results for Various Blend Proportions                                    of Class F Fly Ash and CKD                                                    Misc.    1:1.5   1:1.2   1:1   1.2:1 1.5:1 4:1.sup.1/                         ______________________________________                                        Slump, Inc.                                                                            4.0     4.25    4.5   3.75  3.5   3.0                                Air Content                                                                            4.5     6.0     5.0   4.2   5.5   4.0                                Unit Wt. #/                                                                            146     144     144.6 145.9 144.8 146.0                              ft.sup.3                                                                      Compressive                                                                   Strength                                                                      7 day    3112    3042    3042  3112  3085  3006                                        3112    3042    3254  3072  2971  2959                               Av.      3112    3042    3148  3094  3028  2982                               28 day   4562    4456    4456  4103  4315  4845                                        4562    4173    4386  4386  4244  5199                               Av.      4562    4314    4436  4244  4280  5022                               56 day   5659    5376    5411  5376  5411  6048                                        5624    5586    5659  5482  5341  6437                               Av.      5642    4482    5535  5429  5376  6242                               ______________________________________                                         .sup.1/ Comparative Example: 30% cement replacement, using 329# portland      cement and 113# of Class F Fly ash and 28# of CKD.                       

What is claimed is:
 1. In a composition for producing general purposeconcrete which comprises per cubic yard of the concrete to be produced agenerally homogeneous admixture of about 400-750 lbs. portland cement,about 1600-2000 lbs. coarse aggregate of a size of at least about 3/8inch, and sufficient fine aggregate of a size less than 3/8 inch and upto minor amounts of conventional additives to yield one cubic yard whenthe admixture is combined with water in an amount equal to about 40-70%by weight of said portland cement, the improvement wherein said portlandcement in said admixture is partially replaced with a) Class F fly ashhaving a calcium oxide content up to about 6% and b) cement kiln dust,said Class F fly ash and cement kiln dust being present in amountshaving a weight ratio of about 2:3-3:2 and which When combined equalabout 25% up to about 50% by. weight of said portland cement but notmore than about 200 lbs. of said Portland cement.
 2. The composition ofclaim 1 wherein said coarse aggregate has a size up to about 1".
 3. Thecomposition of claim 1 wherein at least about 30% of said portlandcement is replaced.
 4. The composition of claim 1 wherein said fly ashhas a combined content of silica, alumina and ferric oxide exceeding 70%and a calcium oxide content of less than 10% and said fly ash and cementkiln dust together have a content of silica, alumina and ferric oxidehigher than about 50% but less than about 70% and a calcium oxidecontent higher than 10%.
 5. The composition of claim 1, wherein saidfine aggregate is graduated in size from about #100 up to about #4 sievesize and said coarse aggregate is graduated in size from about 3/8" upto at least about 3/4" size.
 6. A composition for producing generalpurpose concrete which comprises per cubic yard of the concrete to beproduced a generally homogeneous admixture of about 400-750 lbs.cementitious ingredients, about 1600-2000 lbs coarse aggregate of a sizeof a at least about 3/8 inch, and sufficient fine aggregate of a sizeless than 3/8 inch and up to minor amounts of conventional additives toyield one cubic yard when the admixture is combined with water in anamount equal to about 40-70% by weight of said cementitious ingredients,said cementitious ingredients consisting essentially of about 50-75% byweight of portland cement and a combined amount equal to about 25-50% byweight up to a maximum of about 200 lbs of Class F fly ash having acalcium oxide content up to about 6% and cement kiln dust, said fly ashand cement kiln dust having a weight ratio in the range of about2:3-3:2.
 7. The composition of claim 6, wherein said fine aggregate isgraduated in size from about #100 up to about #4 sieve size and saidcoarse aggregate is graduated in size from about 3/8" up to at leastabout 3/4 size.
 8. In a method of making general purpose concrete whichcomprises the step of mixing concrete-making ingredients comprising400-750 lbs. portland cement, about 1600-2000 lbs. coarse aggregate of asize of at least about 3/8 inch, and sufficient fine aggregate of a sizeless than 3/8 inch and up to minor amounts of optional conventionaladditives to yield one cubic yard when the admixture is combined withwater equal to about 40-70% by weight of said cement, the improvement ofpartially replacing said Portland Cement with Class F fly ash having acalcium oxide content up to about 6% and cement kiln dust in a combinedamount equal to about 25% to about 50% of said Portland cement, theClass F fly ash and said cement kiln dust being present in a weightratio of said fly as to said cement kiln dust of about 2:3-3:2.
 9. Themethod of claim 8, wherein said fine aggregate is graduated in size fromabout #100 up to about #4 sieve size and said coarse aggregate isgraduated in size from about 3/8 up to at least 3/4 size.